1. Field of the Invention
The present invention relates to a crystallization apparatus and a crystallization method of semiconductor, and more particularly to an apparatus, a method, a mask and others for generating a crystallized semiconductor film by irradiating a polycrystal semiconductor film or an amorphous semiconductor film with a laser beam through a mask.
2. Description of the Related Art
Materials of a thin film transistor (TFT) used for a switching element or the like which controls a voltage to be applied to pixels of, e.g., a liquid crystal display (LCD) are conventionally roughly divided into amorphous silicon and polysilicon.
The polysilicon has higher electron mobility than that of the amorphous silicon. Therefore, when a transistor is formed by using the polycrystal silicon, a switching speed is increased as compared with a case using the amorphous silicon, and a response speed of a display is also increased. Further, such a transistor can be used as a thin film transistor of a peripheral LSI. Furthermore, using such a transistor can obtain advantages such as a reduction in design margin of any other components.
In case of incorporating peripheral circuits such as a driver circuit or a DAC other than a display main body into a display, constituting these peripheral circuits by the transistors enables an operation at a higher speed.
The polycrystal silicon consists of a number of crystal grains, and it has lower electron mobility than that of monocrystal silicon or crystallized silicon. Moreover, in a small transistor formed by using the polysilicon, irregularities in a crystal grain boundary number at a channel portion are a problem. Thus, in recent years, there has been proposed a crystallization method which generates monocrystal silicon with a large particle diameter in order to improve the electron mobility and reduce irregularities in a crystal grain boundary number at the channel portion.
As this type of crystallization method, there is known “phase control ELA (Excimer Laser Annealing)” which generates a crystallized semiconductor film by irradiating a phase shift mask neighboring a polycrystal semiconductor film or an amorphous semiconductor film in parallel with an excimer laser beam. The detail of the phase control ELA is disclosed in, e.g., “Surface Science Vol. 21, No. 5, pp. 278–287, 2000”.
In the phase control ELA, the phase shift mask generates a light intensity distribution having an inverse peak pattern. In this pattern, a light intensity is minimum or substantially 0 at a point corresponding to a phase shift portion of the phase shift mask (pattern that a light intensity is substantially 0 at the center and it is suddenly increased toward the periphery) is generated. A polycrystal semiconductor film or an amorphous semiconductor film is irradiated with the light having this light intensity distribution with the inverse peak pattern. As a result, a temperature gradient is generated in a fusion area of the semiconductor film in accordance with the light intensity distribution, and a crystal nucleus is formed at a part which is first solidified in accordance with the point where the light intensity is minimum or substantially 0. Then, a crystal grows in a lateral direction (lateral growth) from the crystal nucleus toward the periphery, thereby generating monocrystal grains with a large particle size.
In a prior art, a generally used phase shift mask is a so-called line type phase shift mask constituted by two types of rectangular areas which are alternately repeated along one direction. A phase difference of π (180 degrees) is given between the two different-type areas. In this case, as shown in FIG. 22, a boundary 200 between two different-type areas 201 and 202 having different thicknesses or different phases constitutes a phase shift portion. The polycrystal semiconductor film or the amorphous semiconductor film is irradiated with the light which has been transmitted through such a phase shift mask. The irradiated light has a light intensity distribution with an inverse peak pattern portion RP such that a light intensity is substantially 0 or minimum at a position on a line corresponding to the phase shift portion 200 and the light intensity is one-dimensionally increased toward the periphery.
As described above, in the prior art using the line type phase shift mask, a temperature becomes lowest along a line corresponding to the phase shift portion (boundary 200), and a temperature gradient is generated along a direction orthogonal to a line corresponding to the phase shift portion. Further, it is general that the light intensity distribution (curved line) at a middle portion MP between the two adjacent inverse peak pattern portions RP involves irregular undulations (wave-formed distribution such as that an increase and a decrease in the light intensity are repeated).
In this case, it is desirable for the positional control of a crystal nucleus that the crystal nucleus is generated at a position with a large inclination or a position 210 close to a minimum intensity point in the light intensity distribution with the inverse peak pattern portion RP. However the crystal nucleus may be inconveniently generated at a position 220 with the low light intensity (i.e., an undesired position) in the undulations of the middle portion. Furthermore, even if the crystal nucleus is generated at an undesirable position, the lateral growth which has started from the crystal nucleus toward the periphery tends to stop at a part where the light intensity is decreased near the boundary between the inverse peak pattern portion RP and the middle portion MP.